Polymeric biocomposites

In a similar fashion, others have shown the ability to graft oUgo(Iactone)s to an inorganic surface without a silane-coupling agent, as depicted in Fig. 3.2(f). This was demonstrated through protonation of a tricalcium phosphate (TCP) surface in an aqne-ous solution of phosphoric acid, followed by the in situ surface polymerization of lactones, such as PCL or L-lactide (Kunze et al., 2003). In this surface-modification method, surface-polymerized chains were directly attached to exposed hydroxyl reactive groups inherent on the TCP surface. 

Due to their tunable degradability, biocompatibility, processibility, and versatility, polymeric biocomposites are principal materials investigated for the development of synthetic bone scaffolds, cements, and composites (Porter et al., 2009). As previously defined, a polymeric biocomposite is composed of two or more bulk biomaterials (at least one a polymer) of different phases intended for use in the body. There are an unfathomable number of biocomposites that fit this broad criterion. Classic polymeric biocomposites for orthopedic applications are composed of a solid, synthetic ceramic phase that is osteoconductive or -inductive (Sepulveda et al., 2002) and a biocompatible polymer that was at one stage a liquid. 

An ideal polymeric biocomposite both initially mimics the properties of the native bone tissue it is intended to replace and also remodels to form new bone. Consequently, choosing the appropriate individual components within a biocomposite, and the manner in which they are combined, is critical. The individual components must be biocompatible, biodegradable, and mechanically robust. The ideal polymeric biocomposite must be fabricated in a manner that allows 

Polymeric biocomposites that have utilized techniques to modify the surface of the solid phase before combination with the polymer binder narrow the field. The filler material and polymer binder combinations that fit this criterion and are highlighted in this chapter are paired in Table 3.1. In addition to the surface-modification techniques, what differentiates these biocomposites from one another are the fabrication processes used to combine the polymer and bioactive components and the polymerization of the polymer itself. A general schematic of how surface-modified polymeric 

Liu et al., 1998b,c) Polyethylene glycol and poly(butylene terephthalate) (PEG/PBT) block copolymer (Liu et al., 1997,1998a) Bisphenol A glycidyl methacrylate (BisGMA) (Santos et al., 2002) Polyethylene (Wang and Bonfield, 2001), high-density polyethylene (HDPE) (Sousa et al., 2003) 

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El Kady A.M., K.R.M., El Bassyouni G.T. (2009). Fabrication, characterization and bioactivity evaluation of calcium pyrophosphate/ polymeric biocomposites. Ceram. Int. 

TG-MS), are applied quite rarely, although they may yield useful information for both fabrication (by thermal processing methods) and life-time predictions of polymeric biocomposites. 

Effects of surface modification on polymeric biocomposites for orthopedic applications... 

This chapter will focus on fundamental concepts related to surface modification of materials utilized within polymeric biocomposites for orthopedic applications. For this chapter, orthopedic applications are defined as medical indications or procedures that benefit from utilization of polymeric biocomposites and/or additional implanted therapeutic material to aid in bone regeneration at a localized site. The term surface modification refers to the physical attachment of molecules, predominantly silanes and/or polymers, to the surface of a solid-phase material. Polymeric biocomposites are a class of biomaterials that comprises a biocompatible bulk polymer and a particulated solid phase, often referred to as a binder and a filler, respectively. As there are vast combinations of polymers and solid materials that fit this definition, this chapter highlights solely those combinations that have been utilized for orthopedic applications, in either the acadenuc or the medical industry settings. 

This chapter will not discuss surface modification methods used throughout orthopedic applications not related to polymeric biocomposites. These methods focus on improving the interaction between bulk materials, such as metal implants, and the body through modification of the implant surface. Although this is an important field of smdy, the effects of these forms of surface modification are not applicable to polymeric biocomposites. 

The chapter outUne is as follows. The first section provides a general overview of related orthopedic applications as well as the state of materials currently utilized in clinical settings related to these applications. This background information is followed by common approaches and methods for modil g the surface solid-phase materials in the orthopedic field. Next, an overview of the materials including the types of polymers and solid fillers used within polymeric biocomposites for orthopedic applications is given, as well as fabrication methods. Afterward, the effects of surface... 

Figure 3.1 Images of orthopedic procedures that currently utilize, or may benefit from use of, polymeric biocomposites, (a) Screw augmentation, (b) tibial plateau fracture, and (c) vertebroplasty.

Table 3.1 Surface-mocUfied polymeric biocomposite components...

Figure 3.3 Surface-modified polymeric biocomposites, (a) Schematic of general fabrication of polymeric biocomposites. Cross-section SEM images of biocomposites made with silane modified fillers (b) Ti02/HDPE (c) phosphate glass fibers/PCL (d) modified HA/BisGMA, and silane-i-PCL modified fillers (e) BG/PUR.

Attachment of molecules to the surface of a solid filler in polymeric biocomposites affects a variety of innate properties, particularly those related to the surface of (he filler material. An overview of the surface modification techniques and how they alter specific filler properties is outlined in Table 3.3. The attachment of molecules affects the immediate physical and chemical composition of a surface, which can alter secondary surface properties related to surface interactions, such as wetting, zeta potential, suiface solution reactions including dissolution/degradation, as well as cellular interactions. These primary and secondary properties do not necessarily alter how the filler interacts with polymer binders in a biocomposite setting, but these properties can change the inherent overall properties of the resultant filler. 

Table 3.3 Effects of surface modification on material and polymeric biocomposite properties... 

Solid bioactive fillers in polymeric biocomposites ideally would not only produce an environment conducive to osteoblast activity via ion release, but would also provide a surface conducive for cell attachment, integration, and proliferation. The presence... 

Effects of surface-modified fillers on the properties of resultant polymeric biocomposites... 

Modifying the surface of sohd fillers used in polymeric biocomposites controls the surface properties (both primary and secondary), which affects both the mechanical and physical properties of the resultant polymeric biocomposite as well as its ability to remodel in vivo. An overview of the surface-modification techniques and how they alter the resultant biocomposite properties is outlined in Table 3.3. The fundamental theory of composite design is to obtain physical properties that lie between those of the individual components. As previously outlined, a primary motivator to modify the surface of a solid filler is to inaease adhesion between the solid filler and polymer components, and thus the overall mechanical properties of the biocomposite. This observation has been supported by numerous studies citing an increase in tensile properties. Other overall biocomposite properties that are affected by surface modification of filler components include binding to polymer phase, solid-filler incorporation into polymer binder, water uptake, and degradation. 

The fundamental surface-modification methods applied to solid fillers in polymer biocomposites, such as those previously outlined, are based on techniques and surface chemistries that have been utilized for several decades. More recently, new methods have been applied to the surface modification of solid fillers intended for use in polymeric biocomposites for orthopedic applications. Plasma polymerization forms polymeric materials, such as nanoscale-thick polymer coatings, via partially ionized gas (plasma) (Larranaga et al., 2013 Nichols et al., 2007). This rapid and solvent-free alternative approach to the conventional wet-surface modification processes previously described has several advantages that may be particularly appealing for the... 

As an extension to this surface-modification method, researchers have utilized plasma polymerization of acrylic acid to immobilize biologically active molecules, such as recombinant human bone formation protein-2 (rhBMP-2). rhBMP-2 is a signaling molecule that promotes bone formation by osteoinduction that has been utilized for various orthopedic tissue-engineering applications (Kim et al., 2013). One research group modified a PCL scaffold surface with plasma-polymerized acrylic acid (PPAA) and rhBMP-2 via electrostatic interactions (Kim et al., 2013) (which is outside of the scope of this chapter). This interesting approach may be apphed to the surface modification of solid fillers and provide additional benefits compared to the surface-modification techniques currently utihzed in orthopedic polymeric biocomposite development. The acrylic acid and rhBMP-2-modifled surface showed improved cell attachment and adhesion compared to the surface with acrylic acid alone. The ability to modify the surface of a solid-filler particle in a polymeric biocomposite with a bioactive molecule, such as rhBMP-2, provides a delivery vehicle for the bioactive molecule to the polymeric biocomposite and the eventual implantation site of this biomaterial. Such surface-modification and immobihzation approaches may provide a method to control the release kinetics of attached molecules to the localized bone-defect site. 

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